1. Field of the Invention
This invention relates to a base film for use in a so-called vacuum-deposited magnetic tape, and a magnetic recording medium using the base film.
2. Prior Art
As a high density magnetic recording medium, there is known a magnetic recording medium composed of a non-magnetic support and a ferromagnetic metal thin film formed thereon by physical deposition, such as vacuum deposition or sputtering, or plating.
For example, a magnetic tape on which cobalt (Co) is deposited is disclosed in Japanese patent application laid-open publication No. 147,010/79, and a perpendicular magnetic recording medium using an alloy of cobalt and chromium (Cr) is disclosed in Japanese patent application laid-open publication No. 134,706/77.
The thin film formed by the thin-film formation method, such as vacuum deposition, sputtering or ion plating, is as thin as 1.5 .mu.m or less, and in spite of its thinness, it has magnetic characteristics similar to or higher than those of a coating type magnetic recording medium provided with a magnetic recording layer having a thickness of 3 .mu.m or more. The coating type magnetic recording medium is composed of a base film and a magnetic layer coated on the base film and formed of a mixture of magnetic powder and an organic polymer binder.
In addition, there is the idea that magnetic characteristics such as Hc (coercive force) or remanence ratio of a hysteresis loop doesn't depend on the surface conditions of the base film. As an example of this idea, U.S. Pat. No. 3,787,327 revealed multi-layer structure of Co--Cr formed by vacuum deposition.
On the other hand, a metal thin film magnetic recording medium has such a disadvantage that roughness (surface irregularities) of the base film appears on a metal thin film having an extremely thin thickness, resulting in producing noise upon information reproduction.
In preventing the production of noise, it is desirable that the base film have a surface as smooth as possible. On the other hand, from the aspect of handling, such as winding and unwinding of the base film, the surface of the base film should be rough to some extent since an exceedingly smooth surface of the base film causes deterioration of performance in terms of slippage between adjacent portions thereof, resulting in occurrence of so-called blocking which makes it impossible to use it as a commercial product. Accordingly, the surface of the base film is required to have a certain roughness.
As mentioned above, in a metal thin film-type magnetic recording medium, the smooth surface of the base film is required from the aspect of electromagnetic transducing characteristics. On the other hand, the rough surface is required from the aspect of handling. In the end, the above-mentioned base film must satisfy these two contradictory characteristics.
Further, in the metal thin film magnetic recording medium, travelling ability of the surface on a metal thin film side is important in a practical use. In the case of the coating type magnetic recording medium, for example, the travelling ability is improved by adding a lubricant in a binder. However, such countermeasures are not applicable to a metal thin film magnetic medium. It is, therefore, much difficult to keep a stable travelling ability, and particularly, extremely difficult under high-temperature and high-humidity conditions.
In order to solve the problem, it has been proposed that fine projections be formed on the surface of a base film by employing a fine powder, water-soluble resin and silane coupling agents, as disclosed in Japanese patent laid-open patent publication No. 30,105/87. It has been also proposed that fine projections be formed on the surface of a base film by employing a fine powder and water-soluble resin, as disclosed in Japanese patent application laid-open publications Nos. 30,106/87 and 229,316/84.
As mentioned above, the roughness of the base film surface on which a magnetic layer can be formed is controlled in order to optimize the roughness of the surface of a metal thin film. However, the surface on the other side (the so-called back surface) has not been considered since it is the surface on which the magnetic layer can be formed that has been of primary concern. A surface having a center line average roughness Ra of approximately 4 nm, a ten-point average roughness of approximately 30 nm and a maximum height Rmax of 40 nm is usually considered "a relatively good surface" for the back surface only to prevent the back surface from suffering from deterioration by heat. Heat deterioration, here, means damage and deformation of a base film caused by the heat produced during the formation of a magnetic metal thin film by vacuum deposition. This is because, for example, in the case of applying vacuum deposition, a magnetic metal beam which is produced by the evaporation of magnetic metals is applied.
The roughness of the back surface, which contacts a guide roll in the production process of the film and the magnetic recording medium, largely influences travelling ability. Further, in the case of the above-mentioned relatively good surface, the travelling ability becomes unstable resulting in an increase in production costs of both the base film and magnetic recording medium.